IplImage and cv::Mat are different header types for matrix/image data. They're basically compatible with each other, but IplImage is used in OpenCV's C API, while cv::Mat is used in the new C++ API.

When you decide to use OpenCV's C API, you will mostly use IplImages. Otherwise you will mostly use cv::Mats.

Of course, you can convert IplImages and cv::Mats to each other, e.g.

cv::Mat mat(myIplImage);

In this case, they share the same underlying data, i.e. modifications made through either header will be visible regardless of what header you're using to access the data.

Deep-copy (not only header is "transformed"/copied, but also the underlying data) is possible with

cv::Mat mat(myIplImage, true)

Note that multiple IplImages can also point to the same underlying data, as can multiple cv::Mats.

When working with OpenCV and similar libraries it is important to notice that cv::Mats and IplImages are only "headers" for the actual data. You could say that cv::Mats and IplImages are basically pointers plus important meta information like number of rows, columns, channels and the datatype used for individual cells/pixels of the matrix/image. And, like real pointers, they can reference/point to the same real data.

The most important member is char *imageData;. This pointer references the actual image data. However, the IplImage as a whole contains meta information about the image as well, like number of rows and columns.

The IplImage structure was inherited from the Intel Image Processing Library, in which the format is native. OpenCV only supports a subset of possible IplImage formats, as outlined in the parameter list above.

If you are usin C++ you don´t use IplImage because this only use in a C program, (IplImage is in C API).
In addition if you are usin C you need post Cv before the variables names: "CvMat *" "CvPoint " "IplImage " etc.
It ´s better to use C++ code, you don´t use Cv: "Mat *" etc. but you can´t use the IplImage type, instead of IplImage you use a Mat type for example in findchessboardcorner function.